This invention relates to improvements in electric motors, particularly radial interior permanent magnet motors.
Interior permanent magnet motors are well known. The basic design of a radial interior permanent magnet motor comprises a stator and a rotor that share a common axis. The stator has a number of stator teeth with the spaces between the teeth defining slots through which a set of stator windings pass as they wind around the teeth. These windings of electrically conductive wire are arranged in multiple electrical phases, typically three phases. The rotor comprises a number of permanent magnets which are located below the surface of the rotor periphery, i.e. interior to the rotor. The magnets alternate around the rotor between a North pole facing away from the rotor axis and a South pole facing away. The rotor is typically located inside the stator, so that the largest diameter of the rotor is slightly smaller than the smallest diameter of the stator, giving an air gap between the rotor and stator, so that the rotor is free to rotate.
In use, alternating drive currents are passed through the windings under control of a motor controller. These currents generate a magnetic flux which interacts with the magnets of the rotor, and by varying the currents in each winding as a function of the relative angular position of the rotor and the stator the magnetic flux will cause the rotor to turn to align the poles of the permanent magnets with the magnetic flux created by passing current through the coils. A wide range of control strategies are known, and the operation of such as motor forms a part of the common general knowledge of an expert in interior permanent magnet (PM) motors.
One known phenomenon exhibited by a motor of the kind described above is cogging torque. This is a torque ripple that occurs as the motor rotor rotates due to interaction between the rotor magnets and the stator teeth which are typically steel. The cogging torque is dependent on the position of the rotor. An interior permanent magnet motor having a rotor with an outer periphery of constant radius will exhibit a high degree of cogging torque. This is undesirable as it leads to a jerky movement of the rotor as it rotates, which is especially noticeable at low speeds and low drive currents.
The periodicity of the cogging torque depends on the number of magnets and the number of stator teeth. The lowest cogging torque order in a perfect motor, by which we mean a motor that is without manufacture tolerance, is determined by the least common multiple of rotor pole number, i.e. the number of magnets, and the number of teeth on the stator tooth, i.e.N=LCM(2p,Ns)
where LCM is the lowest common multiple, 2p is the number of magnet poles and Ns is the number of slots, equal to the number of teeth.
However, when manufacture error occurs, the lowest cogging torque order in one mechanical period cannot be calculated according to the equation above anymore. It has to be modified asN=LCM(Nrotor,Nstator)  (1)
where Nrotor indicates the number of periodical parts of rotor and Nstator represents the periodical parts of stator.
For example, a motor with 12 stator teeth and 8 rotor magnets having one tooth radially misaligned will suffer from 8th order cogging torque. A similar motor with 3 stator and 4 rotor manufacturing errors will suffer from 12th order cogging torque. The vast number of possible combinations of manufacturing errors such as deformation or offset of the teeth or poles, or the rotor and stator being out of round mean there can be relatively large variations in the harmonic content of the cogging torque even for small variations between motors of nominally the same dimensions.